The present invention relates to lubrication systems for turbine engines and for associated integrated drive electrical power generator systems and, more particularly, to various outside fluid and lubricant heat exchangers for use in maintaining desired temperatures of the lubricants in such engines and systems.
Lubrication systems for turbine engines and associated equipment, such as a turbofan engine and an associated integrated drive electrical power generator, provide pressurized lubricant, an oil, to lubricate, cool and clean the engine main bearings, gearbox gears, and the like. Similarly, pressurized oil is used for the lubrication of bearings and other parts in an associated integrated drive generator system. During such lubrications, heating of the lubricant is caused to occur due to mechanical energy losses in the lubricated apparatus. Thermal management of such lubricants is very important for continued successful operation of such lubrication systems in the apparatus lubricated thereby.
The amount of heat necessary to be ejected from lubricants in such systems is increasing in evolving engines and associated systems because of the use of larger electrical generators, for instance, in aircraft turbine engines due to increasing consumption of electrical power in the aircraft powered thereby, and because of the advances in aircraft turbine engines such as the use of geared turbofans for such aircraft with large fan-drive gearboxes. Despite the added heat generated by the such modified and expanded equipment, the necessary lubricating oil operating temperature ranges to provide satisfactory lubricating performance have not changed for the most part and, in some instances, the upper operating temperature limits have been reduced.
The lubrication system for a turbofan engine in an aircraft typically has a first heat exchanger providing lubricating oil passing through passageways in that heat exchanger that is cooled by the fuel stream flowing past these passageways. This arrangement permits the lubricating oil to reject heat therein to the fuel being burned by the engine thereby heating that fuel to help recover some of the energy lost in the combustor of the engine.
Because, in some flight situations, more heat is generated in the lubricating oil than is needed for warming the fuel, a portion of the lubricating oil can be forced to bypass the heat exchanger for the fuel and the lubricating oil, and the oil can be directed to a further heat exchanger where the heat therein is transferred to the air in the secondary airstream provided by the fan of the turbofan engine. In a typical arrangement, a duct is provided in the fan cowling through which a portion of the airstream is diverted, or, more recently, provided in a fan duct bifurcation structure, and the air and lubricating oil heat exchanger is placed in this duct so that the lubricating oil passing through passageways in that heat exchanger is cooled by the duct airstream flowing past these passageways in the exchanger. If such additional cooling of the oil is not needed in a flight situation, the air can again be forced to bypass this air and lubricating oil heat exchanger.
An integrated drive generator system that is powered by the associated turbofan engine also has a lubrication system in which the oil used as a lubricant therein is forced by a pump through a heat exchanger where the heat therein is transferred to the air in the secondary airstream provided by the fan of the turbofan engine. Here, too, a duct is typically provided in the generator structure through which a portion of the airstream is diverted with the generator air and lubricating oil heat exchanger placed therein so that the lubricating oil passing through passageways in that heat exchanger is cooled by the duct airstream flowing past these passageways in the exchanger.
Any of the fan airstream that is diverted to pass through the lubricating oil and air heat exchangers in such duct systems may be regulated by some air valve or stream limiting door in the duct containing the exchanger, and the exchanger must be large enough, insofar as assuring that a sufficient part of the cooling engine fan airstream flows over a sufficient amount of lubricating oil flowing in passageways therein, to provide adequate oil cooling for the most extreme preflight or flight conditions encountered, or both. This is true even though this heat exchanger size is not needed for many, or even most, of these preflight or flight conditions. Such a larger sized exchanger correspondingly requires larger fairings about that exchanger leading to a) possible detachment of the fan streams therefrom and the resulting vortex losses absent further preventive measures, b) a larger inlet to the duct possibly resulting in the “spilling” out of incoming air and the accompanying eddy and mixing losses, and to c) a larger range of required motion for the required larger size duct outlet flaps possibly leading to this flap interfering more with the fan airstream passing the outside of the flap when in the range of being nearly fully open to being fully closed. These three consequences, even in an optimally configured arrangement, will result in pressure losses. Thus, such an air and lubricating oil heat exchanger duct based system continually leads to thrust losses in the turbofan engine despite being unnecessary for cooling the lubricating oil in many flight situations.
One arrangement allowing for reduced sizes of lubricating oil and air heat exchangers, and so the ducts in which they are provided, is to add a heat exchanger coupling the lubrication systems in the engine and the generator. An oil and oil heat exchanger can be relatively small because of being a liquid to liquid heat exchanger with a resulting very efficient transfer of heat. This coupling allows the oil and air heat exchangers in each lubrication system to help cool the other since the extreme conditions for needing cooling in one system differ from the extreme conditions for needing cooling in the other. Nevertheless, space for a further heat exchanger must be found to accommodate such an oil and oil heat exchanger. Hence, there is a strong desire for a lubricating oil thermal management system to control fuel and oil temperatures that also reduces such thrust losses and additionally reduces the volume required therefor in the more compact available spaces in advanced turbofan engines and associated equipment arrangements.